Interactions among insect defoliation, insecticide treatments, and growth rate in American and hybrid chestnuts

The American chestnut, Castanea dentata, was once one of the most useful and abundant canopy trees in eastern North American forests. Over the last 200 years, the species has been decimated by two exotic pathogens, Phytophthora cinnamomi and Cryphonectria parasitica, killing millions of trees and reducing surviving Castanea dentata to short-lived sprouts. Cryphonectria parasitica-resistance breeding programs are currently producing advanced backcross generations, which are being compared with pure American chestnut in field tests of growth performance and Cryphonectria parasitica resistance. The Asiatic oak weevil, Cyrtepistomus castaneus, has been identified as a common defoliator of chestnut seedlings in these tests. A study was initiated in eastern Tennessee to quantify the impact of defoliation on growth of American and hybrid chestnut seedlings in 2013 and 2014 and northern red oak, Quercus rubra, seedlings in 2014. Defoliation ratings were used to evaluate the efficacy of the insecticides imidacloprid, acephate, and dinotefuran and a water control, applied via soil drench in June 2013. The abundance and emergence timing of Cyrtepistomus castaneus was monitored throughout the growing season. Defoliation was relatively low and mean percent defoliation ratings did not differ among American and hybridchestnut or northern red oak seedlings. In most months, seedlings treated with dinotefuran or imidacloprid had the least amount of defoliation, suggesting both treatments were effective. However, seedlings given water control only became significantly different from dinotefuran and imidacloprid toward the end of the growing season. No strong correlations between two year growth and percent defoliation occurred implying the defoliation levels that were observed did not strongly impact early growth. Cyrtepistomus castaneus emergence began in early May, peaked in late July through mid-August, and ended by late November. No significant differences were found in spatial patterns of emergence of Cyrtepistomus castaneus suggesting relatively equal distribution of insects on the site.